A variety of mount assemblies are presently available to isolate vehicle vibrations, such as produced during operation of automobile and truck engines and transmissions. One of the most popular mounts today is the hydraulic/elastomeric mount of the type disclosed in U.S. Pat. No. 4,588,173 to Gold et al, issued May 13, 1986 and entitled "Hydraulic-Elastomeric Mount".
The hydraulic mount assembly of this prior invention includes a reinforced, hollow rubber body that serves as both a load supporting means and an integral part of the damping means. A hydraulic cavity partially formed by the body is closed by a resilient diaphragm. This cavity is partitioned by a plate into two chambers that are in fluid communication through a relatively large central orifice in the plate. The first or primary chamber is formed between the orifice plate and the body. The secondary chamber is formed between the plate and the diaphragm.
A decoupler is positioned in the central orifice of the plate and reciprocates and responds to vibrations. The decoupler movement alone accommodates small volume changes in the two chambers. When, for example, the decoupler moves in a direction toward the diaphragm, the volume of the primary chamber increases and the volume of the secondary chamber correspondingly decreases. In this way, at certain small vibratory amplitudes and generally higher frequencies, fluid flow between the chambers is substantially avoided and undesirable hydraulic damping is eliminated. In effect, this freely floating decoupler is a passive tuning device.
In addition to the large central orifice, an orifice track with a smaller flow passage is provided extending around the perimeter of the orifice plate. Each end of the track has an opening; one communicating with the primary chamber and the other with the secondary chamber. The orifice track therefore provides the hydraulic mount assembly with another passive tuning component, and when combined with the freely floating decoupler provides at least three distinct dynamic operating modes. The particular operating mode is primarily determined by the flow of fluid between the two chambers.
More specifically, small amplitude vibrating inputs such as from smooth engine idling or the like, produce no damping due to the action of the decoupler, as explained above. Large amplitude vibrating inputs produce high velocity fluid flow through the orifice track, and accordingly a high level of damping force, and desirable smoothing action is obtained. The high inertia of the hydraulic fluid passing through the orifice track contributes to the relatively hard mount characteristic in this mode. A third or intermediate operation mode of the mount occurs during medium amplitude inputs resulting in lower velocity fluid flow through the orifice track, generally resulting in a medium level of damping. In each instance, as the decoupler moves from one seated position to the other, a relatively limited amount of fluid can bypass the orifice track by moving around the sides of the decoupler to smooth the transition between the operational modes.
While this mount has proven highly successful in isolating engine/transmission vibration, it has shown a tendency to show a very sharp increase in dynamic rate at frequencies substantially above the resonant frequency of the engine/transmission. Research has indicated that the characteristics of fluid flow through the orifice track change in this situation and results in the tendency toward limiting fluid flow between chambers. It is theorized that this situation creates an internal pressure build-up, resulting in the undesirably high dynamic rate. Various designs have been tested to remedy this problem.
Other recent developments in hydraulic mount technology have lead to the advent of electronic control of the damping characteristics of the mount. Such a hydraulic mount is disclosed in U.S. Pat. No. 4,789,143 issued Dec. 6, 1988 and assigned to the assignee of the present invention. The mount described in U.S. Pat. No. 4,789,143 represents a modification of previous mounts in that it provides variable damping levels in response to sensed vehicle operating conditions. This active tuning of the mount is clearly a more sophisticated approach and has found general acceptance among engineers and others as an advancement in the art. The tuning is accomplished in this particular embodiment by the use of an infinitely variable sliding gate for selectively varying the size of the opening to the orifice track between the two chambers. By varying the opening size, the flow of damping fluid and thus the damping action of the mount can be changed.
Another approach to active tuning involves providing an inflatable bellows in the primary chamber of the mount. Such a mount is described in U.S. application Ser. No. 240,688, filed Sept. 6, 1988 and entitled "Hydraulic Engine Mount With Bellows Tuning". Transducers and an electronic controller regulate the flow of air into and out of the bellows in order to control the damping effect the mount.
Not only have these prior art mounts with active control proven to be successful in further modulating the response of the mount of vehicle operating conditions, but they can be programmed to operate in a manner particularly adapted to the vehicle configuration in the particular component, such as a motor or transmission, being damped. However, the disadvantage of these new and more sophisticated systems is the relatively higher cost of manufacturing and maintenance. Thus, it would be desirable to improve the operating response of a tunable hydraulic mount with an alternative approach to these prior art systems, and particularly the active systems. It would also be desirable to build in a control of internal pressure build-up created at operating frequencies substantially above the resonant frequency of the engine/transmission to maintain the dynamic rate of the amount at an acceptable level.